Iron Transport in Mycobacterium smegmatis: Uptake of Iron from Ferriexochelin

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Abstract
Summary: Exochelins are a group of extracellular iron chelators produced by mycobacteria. Iron uptake by washed suspensions of iron-deficiently grown Mycobacterium smegmatis from 55Fe(III)-exochelin fractions (at about 1 μm) was greatest from the fractions containing the compounds that naturally predominate in culture filtrates. Uptake from the major fraction, as well as from combined exochelins, had a Km of about 6 μm and was unaffected by the presence of a large excess of desferriexochelin; it was inhibited by more than 90% by electron transport inhibitors, uncouplers of oxidative phosphorylation, thiol reagents and by anaero-biosis and low temperature. Uptake of iron from 55Fe-salicylate, which is mediated by myco-bactin, was insensitive to these inhibitors and a 10-fold excess of ferric salicylate did not inhibit 55Fe-exochelin uptake. Thus mycobactin is probably not involved in the transport of iron from ferriexochelin at physiological concentrations. The rate of uptake of iron from 55Fe-exochelin into iron-sufficiently grown cells, which contain less than 0·05% of the concentration of mycobactin found in iron-deficiently grown cells, was only slightly lower than the rate of uptake into iron-deficiently grown cells.